Generally, hot-forged copper alloy parts are excellent in terms of hot forgeability, machinability, antibacterial properties and bactericidal properties, have high strength, favorable corrosion resistance and favorable conductivity, and are thus used for a variety of industrial machinery and facilities, mechanical parts in automobiles, and electric components. In addition, hot-forged copper alloy parts are used for members such as valves, ball valves, joints, joints and connection tools for crosslinked polyethylene tubes, tube joints and connection tools for crosslinked polybutene tubes, connection tools for water supply and drainage, hose nipples, connection tools for gardening hoses, connection tools for gas hoses, lids for water meters, water faucets, hydraulic containers, nozzles, sprinklers, flare nuts, nuts, water supply and hot-water supply facilities, air-conditioning facilities, containers, connection tools and devices for fire protection facilities and gas facilities, containers and devices through which water, warm water, refrigerants, air, town gas and propane gas pass, and the like.
Particularly, copper alloys have favorable strength, corrosion resistance, antibacterial properties and bactericidal properties, and are thus widely used for potable water-related members. However, since joints, connection tools, faucets and the like, which are potable water-related members, are tubular bodies having a hole portion to allow potable water to pass through, while copper alloys have excellent hot workability and excellent hot forgeability, with techniques of the related art, a copper alloy can be made only into a shape that is far from a near net shape (a shape close to a complete shape), there are problems in that a number of burrs and the like are caused such that the forging yield rate decreases, the cut amount after hot forging is large, and, sometimes, the corrosion resistance or the strength decreases since copper alloys are forged at a high temperature.
The above-described hot-forged copper alloy refers to a body obtained by melting a raw material, casting the raw material into an ingot, cutting a hot-extruded round rod into a predetermined length, then, hot-forging the round rod, and cutting the hot-forged material into predetermined dimensions. Examples of a material for the rod material are mainly based on JIS H 3250, and include forging brass rod C3771 (typical composition: 59Cu-2Pb—Zn(remainder)) that is excellent in terms of hot forgeability and machinability and copper alloy materials obtained by substituting Pb in C3771 with Bi in order to satisfy a recent requirement of the removal of Pb. Particularly, in a case in which excellent dezincification corrosion resistance is required, a forging brass rod which has an increased copper concentration in a range of 61 mass % to 63 mass %, contains 0.5% to 2.5% of Pb, and has dezincification corrosion resistance, and a forging brass rod which is obtained by substituting Pb in the above-described forging brass rod with Bi and has dezincification corrosion resistance are used.
However, when a round rod material is hot-forged as in techniques of the related art, naturally, it is not possible or at least not easy to make the round rod material into a tubular shape, that is, to make a hole portion. In addition, even when attempts are made to decrease the thickness of a portion in which a hole is to be formed in order to increase the forging yield rate, there is a limitation due to a forging load. Furthermore, since a large proportion of deformation energy being added to a forging material is consumed for the molding of the hole portion, it is not possible to mold the shapes of portions other than the hole portion into a predetermined shape. Particularly, in a case in which a forged part has a large aperture (hole diameter or inner diameter), a large outer diameter, and a small thickness, it is difficult to mold a forging material into the near net shape. While a forging facility having a large forging capacity can somewhat decrease the thickness of a portion in which a hole is to be formed and the thickness of a thick portion, there is a limitation in decreasing the thickness. In addition, it is needless to say that the forging facility having a large forging capacity is expensive, and the energy cost for forging further increases since it is necessary to increase the power. In a case in which it is not possible to mold a forging material into a predetermined shape, since the amount of a material being used increases, the material cost significantly increases, and, at the same time, the cut amount increases, the material needs to have improved machinability, and a necessary time for a cutting process also increases.
Hitherto, there have been cases in which the above-described forged copper alloy part having a hole portion is produced using cast metal from the viewpoint of the yield rate. However, there are problems in that cast metal includes a number of defects, has a poor dimensional accuracy, low strength, poor ductility and poor productivity, and is produced in a poor working environment.
Due to what has been described above, there is a demand for a hot-forged copper alloy having a hole portion, that is, a tubular hot-forged copper alloy which can decrease energy consumption by using a low power forging facility that does not require a large amount of facility cost, has a favorable forging yield rate, that is, does not require a large amount of material cost, and has a near net shape that is close to the final finished shape and dimensions.
Regarding the material, in a case in which a finished product is produced using a hollow member, that is, a tubular member through an ordinary hot forging method, it is not possible to mold the member into the near net shape as described above. That is, since it is not possible to make a hollow portion, and portions other than the hollow portion can be molded only into dimensions larger than the predetermined dimensions, the cut amount for achieving the shape of the finished product increases. As a result, as forging materials, there is a demand for a copper alloy having excellent machinability. In order to improve the machinability of the copper alloy, generally, Pb is added, and at least 0.5 mass %, often, 1 mass % or more, and approximately 2 mass % of Pb is added. However, since Pb is harmful, particularly in potable water-related members, the content of Pb is preferably set to 0.3 mass % or 0.2 mass % or less, and it is necessary to suppress the amount of Pb to an extremely small extent. It is needless to say that, when global environmental issues are taken into account, it is also necessary to suppress the use of harmful Pb to an extremely small extent in forged parts being used in potable water-irrelevant fields.
However, since Pb which has a machinability-improving function rarely forms a solid solution in copper alloys, when the hot forging temperature is outside the optimal temperature range, copper alloys are easily cracked during forging. While there is a hot forging copper alloy in which Pb is substituted with Bi, the copper alloy is intended to improve the machinability of copper alloys, and, since Bi is slightly inferior to Pb in term of the improvement of the machinability of copper alloys, a larger amount of Bi is required. Bi-containing copper alloys are more sensitive to cracking during hot forging than Pb-containing copper alloys, and thus there is a problem in that Bi-containing copper alloys have poor hot deformability. Therefore, in the case of Bi-containing copper alloys, it is necessary to set the temperature of hot forging in a narrow range or to increase the thickness of forged parts. In addition, there are problems in that Bi-containing forged parts have poor ductility and low toughness, and the forged parts become embrittled at a temperature in a range of 130° C. to 300° C.
Furthermore, when a hot forging brass rod which has a Cu concentration in a range of 57 mass % to 59 mass % and contains Pb or Bi is hot-forged, a large amount of β phase remains in a forged part, and the corrosion resistance is poor. In Cu—Zn—Pb or Cu—Zn—Bi alloys which have a Cu concentration set to approximately 61 mass % or more and have improved corrosion resistance, when hot deformation resistance increases, the hot deformability deteriorates at the same time. When the copper concentration is high, it is difficult to produce forged parts having the near net shape, the shape of forged parts become complicated, and the moldability and cracking become serious issues as the thickness decreases.
There is a desperate demand for the production of hollow hot-forged copper alloys having the near net shape that is close to the final finished shape and dimensions in which a copper alloy is hot-forged in a single process using a low power forging facility so as to decrease energy consumption, and the cost is reduced by preventing the occurrence of cracking during hot forging, increasing the forging yield ratio, and decreasing the amount of materials being used. When it is possible to mold a copper alloy into the near net shape, the cutting amount decreases, and therefore excellent machinability is not required, that is, it is possible to suppress the content of harmful Pb or Bi having an uncertain stability to become the minimum. Furthermore, there is another desperate demand for a tubular forged part which has excellent corrosion resistance and high strength so as to be further downsized.
In addition, forged brass parts which are intended to improve corrosion resistance, machinability and productivity are known (for example, refer to Patent Document 1). However, in the forged brass parts, it is not possible to forge tubular forged parts into the near net shape.